abstract
Seven prototypical microporous titanosilicates have been studied by multinuclear solid-state NMR (SSNMR) spectroscopy, representing four typical Ti environments: square-pyramidal TiO5 units (natisite, AM-1, ETS-4), edge-shared brookite-type TiO6 chains (AM-4), cubane-type Ti4O16 clusters (sitinakite, GTS-1), and corner-shared TiO6 chains (ETS-10, ETS-4). Ti-47/49 SSNMR spectra at 21.1 T are related to the coordination, crystal symmetry, and local environment of Ti. Distortions in TiO bond lengths and OTiO coordination angles are reflected via C-Q(Ti-47/49) values that range from 8 to 16 MHz. Several titanosilicates feature axially symmetric Ti-47/49 electric field gradient (EFG) tensors that permit facile spectral assignment and detection of deviations in local symmetry. This study uses Si-29 NMR experiments to assess phase purity and crystallinity. Na-23 NMR is used to probe the location and mobility of the sodium ions in the framework. The potential of K-39 SSNMR for investigation of extra-framework counter cations is demonstrated by ETS-10, with increased spectral resolution and enhanced sensitivity to changes in local environment versus Na-23 experiments. Plane-wave DFT calculations predicted Ti-47/49 NMR parameters assisting in spectral assignments and help correlate Na-23 and Si-29 NMR resonances to crystallographic sites. The approach described in this work should promote further SSNMR investigations of microporous solids, such as titanosilicates, with unknown or poorly defined structures.
keywords
METAL-ORGANIC FRAMEWORKS; SODIUM TITANIUM SILICATE; ION-EXCHANGE PROPERTIES; MAS NMR; CRYSTAL-STRUCTURE; LOCAL-STRUCTURE; CHEMICAL-SHIFT; POROUS TITANOSILICATE; AQUEOUS-SOLUTIONS; ADIABATIC PULSES
subject category
Chemistry; Science & Technology - Other Topics; Materials Science
authors
Xu, J; Lucier, BEG; Lin, Z; Sutrisno, A; Terskikh, VV; Huang, YN
our authors
Zhi Lin
Principal Researcheracknowledgements
Y.H. thanks the Natural Sciences and Engineering Research Council of Canada (NSERC) for a Discovery Grant and a DAS award. Funding from the Canada Research Chair program is also gratefully acknowledged. Access to the 900 MHz NMR spectrometer and CASTEP software for select calculations was provided by the National Ultrahigh-Field NMR Facility for Solids (Ottawa, Canada), a national research facility funded by the Canada Foundation for Innovation, the Ontario Innovation Trust, Recherche Quebec, the National Research Council Canada, and Bruker BioSpin and managed by the University of Ottawa (http://nmr900.ca).